1. Field of the Invention
This invention relates to a process for improving material thickness distribution within a molded bottle and the bottle made with the process. The bottle has a closed base or bottom, an open-end, a longitudinal axis, and a generally square, generally rectangular, or generally oval or other noncircular cross-sectional shape typically perpendicular to the longitudinal axis. In particular, the process is for improving material thickness distribution within the bottle base portion or bottom. The bottle base portion contains a chime, contact surface, and push-up region.
The process relies on a mold-cavity base or bottom region of a blow-molding tool modified with at least one generally straight standing rib. The rib (or two or more parallel ribs) alters mold-cavity bottom geometry that redirects material flow during bottle manufacture allowing better placement and distribution of material that in turn minimize unwanted shape distortions in the molded bottle that could otherwise occur.
The invention is suited for bottles made of polyester materials, such as polyethylene terephthalate (PET), or other polymeric materials. The invention is further suited for bottles generally made with an injection based manufacturing process or its equivalent.
2. Description of the Prior Art
Processors generally make bottles from a relatively hot pliable parison or preform using one of several well-known process technologies for making, heating or re-heating the preform, and forming the preform into the bottle. Air pressure inflates this hot pliable preform against a relatively cool cavity surface within the blow-molding tool to form the bottle configuration having approximately the same surface area and shape. Usually, the blow-molding tool is metal and typically aluminum. A molding technician sizes the preform so that, once inflated, the bottle has an appropriate wall thickness and reasonably uniform material thickness distribution throughout its surface.
Achieving good material thickness distribution using the injection based manufacturing process featuring an injection-molded preform is relatively easy for a bottle with a substantially circular cross-sectional configuration. An injection-molded preform is generally a tube with a longitudinal axis, a circular cross-sectional shape perpendicular to the longitudinal axis, and having a sidewall with a substantially uniform material thickness distribution, one open end, and one closed-end. The longitudinal axis of the preform before inflation typically will coincide with the longitudinal axis of the molded bottle made from that preform.
In general, industry uses two injection based blow-molding process technologies. In the first process, preform length is approximately the same as bottle height. Before inflation, the preform closed-end is adjacent or in close proximity to the mold-cavity section forming the bottle push-up. In the second process, preform length is substantially less than bottle height. Before inflation, a stretch-rod stretches the hot pliable preform in an axial direction corresponding with the preform longitudinal axis, generally pinning the preform closed-end against the mold-cavity section forming the bottle push-up before air pressure completely expands the preform in other directions. The industry generally refers to this second process as xe2x80x9cstretchxe2x80x9d blow molding or xe2x80x9cbiaxial molecular orientationxe2x80x9d blow molding. The stretch blow molding process is particularly suited for manufacturing bottles of PET polymer materials.
While the technician strives for the uniform material thickness distribution in the molded bottle, compromises are still often necessary. Regions within the push-up, for example, will tend to have a relatively thicker wall section than most other regions of the molded bottle.
Bottles with generally rectangular or oval cross-sectional shapes, shapes with its inherent major or primary axis and minor or secondary axis generally perpendicular to the bottle longitudinal axis, are often difficult to blow-mold when made with the injection-molded preform. Regions of the inflating preform that must move and stretch a greater distance in a direction generally corresponding with the major or primary axis of the bottle cross-sectional shape will tend to thin more than regions that move in a direction generally corresponding with the minor or secondary axis. Consequently, material thickness distribution is not uniform. The bottle wall thickness adjacent to the ends of the primary axis will tend to be thinner than the bottle wall thickness adjacent to the ends of the secondary axis.
Molding technicians have a number of techniques to improve the material thickness distribution of rectangular or oval bottles, that is, techniques to establish a reasonably uniform material thickness distribution. One approach involves changing how quickly selected regions within the preform stretch by changing the material temperature in that region. A slightly cooler preform region in the preform sidewall will tend to resist and stretch less than adjacent warmer regions. Aligning the cooler preform regions with corresponding areas of the bottle that tend to have an otherwise relatively thinner wall thickness will consequently stretch less thus improving material thickness distribution uniformity.
While this approach, sometimes known as xe2x80x9cheat profiling,xe2x80x9d is effective for improving material thickness distribution uniformity within the sidewall, it is generally not effective for improving material thickness distribution uniformity within the push-up and the molded bottle base or bottom portion. This ineffectiveness is primarily for two reasons.
First, the preform closed-end, the region that forms the bottle base and push-up and having a generally hemispherical shape, is relatively small. While it is feasible to heat profile the entire closed-end to a specific temperature, it is not practical, because of its small size, to heat profile sub regions within the closed-end. Consequently, the heat profiling of the entire preform closed-end region is a compromise generally favoring a need for greater movement in the direction corresponding with the major axis of the bottle cross-sectional shape.
Second, the wall thickness in an area of the push-up surrounding the longitudinal axis of the molded bottle remains relatively thick because the preform closed-end region of the inflating preform has little opportunity to stretch or thin before contacting the relatively cool cavity surface of the blow-mold tool forming the bottle configuration. The wall thickness of preform areas initially contacting portions of the bottle cavity surface will not significantly thin further as the remainder of the inflating preform continues to stretch and come in contact with remaining portions of the bottle cavity surface.
Consequently, the lack of effective heat profiling and the lack of sufficient stretch or thinning of the preform with its circular cross-sectional shape causes the material distribution surrounding the longitudinal axis of the bottle push-up and bottom to have a predominantly thick circular character. The molding process for bottles having a generally square, rectangular, or oval cross-section places this thick circular material distribution within the push-up and base having a corresponding square, rectangular, or oval character.
Overall, the relatively thick areas of the molded bottle tend to cool during manufacture at a slower rate. Consequently, the material within these thick areas is prone to warp and distort. Furthermore, molding technicians, attempting to increase production output, will often remove the bottle from the blow-mold tool before thick areas have sufficiently cooled risking additional distortion of those areas.
When the bottle stands in a typical upright fashion, a region of the base or bottom contacts a supporting surface. The distortions from differences in wall section thickness are generally not a problem with bottles having the circular cross-sectional configuration because these distortions are generally contained within the push-up and out-of-sight. The bottle base portion contacting the supporting surface is usually unaffected.
However, the distortions can create significant problems with bottles having the generally non-circular cross-sectional configuration because the distortions can often extend into the base or bottom contact surface. Sometimes the distortions can extend through the contact surface in the chime. At best, this creates a minor aesthetic problem. At worst, this can alter bottle standing stability by creating a condition molding technicians refer to as a xe2x80x9crocker bottom.xe2x80x9d The rocker bottom condition can create a bottle stability problem on high-speed filling lines, particularly before the bottle filling station.
The process discovered by the inventors permits better placement and control of material thickness distribution within the bottle base or bottom thus minimizing unwanted base distortions. Until this invention, often molding technicians minimized distortions by reduced blow-molding machine output to allow more time for thick areas to cool. This reduces productivity and increases molding cost.
The typically metal blow-molding tool used to manufacture the bottle has several components. One of these components is the mold-cavity base section or region that contains the cavity surface that ultimately forms the base and push-up portion of the molded bottle.
In the case of the noncircular bottle cross-sectional shape, the inventors add to the cavity surface that forms the push-up at least one standing rib-like projection generally parallel to the major axis. Adding these rib-like projections increases push-up surface area and increases surface distance the expanding preform must traverse particularly in the direction generally corresponding with the minor axis. Although not necessarily equal, surface distance in this minor axis direction now approaches the surface distance in the direction corresponding with the major axis.
In addition, the inventors believe the standing rib-like projections slightly alter preform inflation dynamics during bottle manufacture. Because the preform tends to expand first in a direction of least resistance, the rib-like projections tend to initially channel movement of the expanding parison at the base in the direction generally parallel to the rib-like projection thus encouraging additional material to flow in that direction before flow occurs over the rib-like projections in the direction generally parallel to the minor axis. In other words, the rib-like projections briefly block material flow. The standing rib-like projections cause the closed-end of the preform to stretch differently in the direction parallel to the rib-like projection thus favorably altering material thickness distribution.
The relatively thick area of the bottom of the molded bottle is now contained within the push-up minimizing a likelihood of unwanted distortions extending into the base contact region or surface. With less risk of unwanted distortion, molding technicians are now able to increase blow-molding machine output.
The rib-like projections create rib-like grooves in the bottle push-up. Two of these rib-like grooves are particularly useful for bottles having a symmetrical rectangular or oval cross-sectional shape. The inventors anticipate that three or more generally parallel rib-like grooves would be appropriate for some applications of the technology. In the case of three, the middle projection will generally correspond to the major axis.
For bottles having the generally square cross-sectional shape, the inventors anticipate that two pairs of rib-like grooves would be appropriate for some applications of the technology. In this case, a pair of rib-like grooves, each set parallel to the major/minor axes.
The inventors also anticipate that one rib-like projection may be appropriate for certain situations where the cross-sectional shape of the bottle is non-symmetrical in character. For example, a cross-sectional shape resembling the profile of a xe2x80x9ckidney bean,xe2x80x9d that is, having a generally convex shape on one side and a generally concave shape on the opposite side. The one rib approach is appropriate when the relatively thick bottom material would otherwise favor the concave side over the convex side.
In practice, during initial molding trials of the tooling, the inventors make small adjustments to the shape of the standing rib-like projections on the mold cavity surface. These adjustments (usually involving subtle changes in rib taper, relative height, smoothness, roundness, and length) help optimize the material thickness distribution of the blow-molded bottle. Furthermore, to simplify these adjustments, the inventors make the rib-like projections xe2x80x9cmetal safe,xe2x80x9d that is, slightly larger than necessary, to allow relatively easy removal of metal while adjusting the shape. Rib-like projections that are too high or too sharp will tend to overcompensate and to over-thin material thickness. Once the molding operators optimize the shape of the standing rib-like projection, mold makers can easily duplicate the shape in other duplicate mold tooling cavities.
The preferred blow-molding process for improving material thickness distribution within a bottle and in particular a bottle base portion having a longitudinal axis and a noncircular cross-sectional shape generally perpendicular to the longitudinal axis of the bottle comprising the steps of heating a preform with a closed-end and a longitudinal axis; positioning the preform in a mold-cavity of the bottle; the mold-cavity having a mold-base region with at least one generally straight standing rib; expanding the preform with air pressure against the standing rib causing the expanding preform to traverse a relatively longer surface distance across the standing rib; expanding the preform against the mold-base region and the mold-cavity to form the bottle; allowing the bottle to cool; and removing the bottle from the mold-cavity. The mold-cavity has a longitudinal axis that corresponds with the longitudinal axis of the bottle made from the mold-cavity and the longitudinal axis of the preform positioned initially to generally correspond with the longitudinal axis of the mold-cavity.
The blow-molding process can also comprise positioning the closed-end in close proximity to the mold-base region before the preform completely expands.
The blow-molding process can also comprise the step where the expanding preform expands against the standing rib causing material flowing from the closed-end to be momentarily directed along the standing rib before the expanding preform completely traverses the standing rib.
The blow-molding process can also comprise the step where the expanding preform expands in the mold-cavity for a bottle having either a generally square, rectangular, or oval cross-sectional shape. The cross-sectional shape has a major axis and a minor axis generally perpendicular to the longitudinal axis of the bottle.
The blow-molding process can also comprise the step where the expanding preform expands against the mold-base region having at least a pair of standing ribs. Each rib of the pair of standing ribs is generally parallel to and positioned on an opposite side of the major axis.
The blow-molding process can also comprise the step where the expanding preform expands against the pair of standing ribs causing material flowing from the closed-end to be momentarily directed non-radially along each rib of the pair of standing ribs and between the pair of standing ribs in a direction generally biased toward or corresponding to the major axis before the expanding preform completely traverses each rib of the pair of standing ribs.
The blow-molding process can also comprise the step where the expanding preform expands against a third standing rib, parallel and between the pair of standing ribs.
The blow-molding process can also comprise the step where the expanding preform expands in the mold-cavity for a bottle having a generally kidney bean cross-sectional shape with a generally convex side and a generally concave side. The cross-sectional shape has a primary axis and a secondary axis generally perpendicular to the longitudinal axis of the bottle. The standing rib is substantially parallel to the primary axis and between the primary axis and the generally concave side.
The blow-molding process creates a base portion within a blow-molded bottle having improved material thickness distribution and where the base portion merges with a bottle sidewall and the bottle sidewall merges with an open end. The base portion has a noncircular cross-sectional shape, a chime; a contact surface merging with the chime, a push-up region merging with the contact surface, and at least one generally straight rib-like groove within said push-up. The rib-like groove is substantially parallel to the primary axis of the cross-sectional shape of the bottle.
From the following description of the preferred embodiment, the appended claims, and the accompanying drawings, additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates.